Ab,
It's not my intention to get into a brand war here & I won't.

I have several Smiths & I have several Rugers, both put out fine revolvers.
In the .357 caliber, keeping it as much of an Apples/Oranges comparison as possible, the older Security-Six family was roughly Ruger's version of the Smith K-Frame family (Model 19, Model 66).
The current GP-100 family is Ruger's counterpart to the Smith L-Frame series.
The Smith design (in J, K, L, and N frames) derives from about 1899, during an era when smokeless powder was still relatively new, and was based largely on the .38 Special load and pressures in what is now essentially the K-Frame size revolvers. It was able to be upgraded to handle .357 Magnum pressures in the 1950s by the use of better heat treatments in the steel more than improvements in the design, and the Model 19 .357 was born. Previously, S&W's .357s were the Models 27 & 28, built on the larger N-Frame to handle the pressures.
The Ruger Security-Six was designed in the early 1970s from the ground up to handle .357 Mag pressures, it was not adapted from an older low-pressure design. It, and the current GP series, relocated the bolt stop notches in the cylinder away from TDC (the thinnest point in the steel on each chamber) where the Smith notches are still located. This creates a stronger chamber, and while you'll hopefully never encounter an overloaded factory round, the thinnest part of the chamber wall (outside wall section) is what tends to blow if you do fire an over-loaded round. The Ruger chamber walls are simply stronger at this point. Whether that's an issue in actual use or not is relatively unimportant for most shooters, but the Ruger cylinder is generally regarded as being stronger than the Smith by people who understand the difference.
Elsewhere, the crane/yoke on the Rugers (Security-Six & GPs) is dimensioned better to withstand magnum stresses, the section of the frame where the barrel threads into & the barrel shank itself are beefier, and so on.
I'm not an engineer & can't give you exact angles & dangles, but in my line of work I've talked to people who are (including a former Ruger design engineer) along with a number of very good gunsmiths.
When I was having a Smith customized by one of the leading custom shops in the country a couple years back, the guy working on it told me he's never seen a Ruger DA revolver out of time, while the Smiths will need work eventually if shot a lot. Emphasis on A LOT. I'm also seeing timing issues on brand new Smiths that should not be there, but that's a separate issue.
This echoes what I hear elsewhere from people who work on both brands.
Jan Libourel, editor of Gun World magazine, has told the story of his own Smith Model 19. If I recall the figures correctly, he's put about 9,000 rounds through it & had to rebuild it twice. Frame stretching ,forcing cone cracks, and going out of time are the most common problems with extended use in the K-Frame Model 19 .357s, something you very rarely hear about with the older Security-Sixes that competed with them.
Currently, since S&W has dropped most of the K-Frame .357s in favor of the L-Frame (durability issues), and Ruger has dropped the Security-Six in favor of the GP (ease of manufacturing issues), those would be the ones to look at on the new market now.
Again, while the S&W L-Frame has been beefed up overall (in larger "intermediate" frame size) and the frame/barrel attachment area is thicker and stronger than on the old K-Frames, the largest impediment to longevity remains the adherence to the original low-pressure design produced in a low-pressure era and adapted to higher pressures much later on.
The Ruger .357s, as with their .44 Magnums, were all designed from the git-go specifically to withstand the stresses imposed on frame, barrel & cylinder by those calibers, as well as the regular strain & wear caused by the normal interaction of moving parts against each other.
It's not just a matter of forged vs cast, it goes much farther than that.
Same applies, though it's off-topic from the original question, to the two companies' .44 Mag DA revolvers.
The Smith N-Frame was originally built for LOW-PRESSURE big-bore loads in the .44/.45-caliber range. With better heat treating & steels it was upgraded to handle the .357 Mag in 1935, and again to the .44 Mag in 1955 by adding weight in key areas.
The Ruger Redhawks, on the other hand, were not adaptations of 1899-1905 technology & manufacturing methods, and were built from the ground up in modern times (Redhawk 1978, Super Red 1987) using modern manufacturing technology to create guns not only capable of handling .44 Mag pressures, but overly built to do so and fully capable of handling more powerful calibers that will literally blow up a Smith N-Frame.
The Ruger's crane/yoke arrangement is also considerably heftier than the Smith counterpart, which enables the Ruger to deal with the .44 Mag's pressures much better & much longer than any Smith, even with the relatively recent "Endurance Package" upgrades to the Smith .44s.
What does all that translate to for today's shooter?
Back to the original .357 question, it does depend largely on projected use and personal preference.
Ammunition type is a factor. The hotter 125-grain jacketed hollowpoints are harder on the forcing cone than most 158 & 180 grainers. Extended use will erode the cone faster, in new guns the Ruger may have a slight advantage there with more material in the walls. Certainly either of the two Ruger models (old or new) will resist cone cracking and frame stretching longer than the older K-Frame Model 19 or stainless Model 66 with Magnum loads.
Extended firing is a factor. Aside from the forcing cone & frame stretching issues, dimensions & surface contact points are designed in the Rugers to be large & sturdy, and if you fire 10,000 rounds (an arbitrary figure) of full-bore .357 Magnum loads through a brand new 686 and a brand new Ruger GP, I'd be astounded if the Smith did not need some serious work, while I would not be astounded to see that the Ruger didn't.
The obvious question there is how much do you intend to shoot the gun?
For most, as in the case of the S&W Model 29 .44 Mag, the great majority of shooters will never put anywhere near that many rounds through a given gun, in which case longevity is less important. But, as those who used their older Model 29s for silhouette competition and other hard-use applications found, the 29s simply did not hold up as well as the Ruger Redhawks over the long run.
Personal preference is a major factor. Smiths are more refined externally in terms of appearance, "look nicer" for many, and they can be tuned to a very fine trigger pull. They're very good guns & capable of lasting a lifetime with excellent performance, again depending on what you shoot in them & how much you do it.
Rugers are more blocky externally, don't have quite the refined angles and curves, and also tend to show more abrupt edges here & there. Smiths traditionally have had a better bluing job, although that's not the case as much as it once was. Ruger triggers can also be tuned quite nicely.
Accuracy can be comparable between the two brands.
Define your needs & your preferences, and go either way.

I started out policing in 1976 with a Ruger Security-Six in .357 Magnum, and carried a Smith 25-5 in .45 Colt in uniform later on. I still have both.
I have bet my life on both brands, had both brands heavily customized & am very satisfied with the results.
In no way do I suggest you buy either one over the other, but I do disagree with the statement that the Ruger revolvers are not stronger (stronger being defined as both physically stronger as well as more durable over high-mileage use) than the Smiths.
S&W's biggest wall in keeping up with the market times is that they're stuck with the original designs & can only keep on trying to find new ways to upgrade them (largely in materials & configurations), while Ruger didn't have that barrier & wasn't bound to an old action type.
Colt tried to bridge the Old/New gap several years ago when their own classic action (also derived from the early 1900s when labor was cheap) got too expensive to produce and the company developed newer action types. You may note that Colt no longer makes any DA revolvers.
Anyway- This says it for me, and I won't argue the point.
Denis